Recycling of post consumer paper products has become much more important in the last few years. Growing concerns about the environment, alternatives to solid waste disposal, increasing consumer demand for quality recycled papers, and various state and federal laws that mandate levels of recycled fibers all have combined to increase the use of recycled paper in paper making. See Paper Recycling, edited by K. L. Patrick, Miller Freeman Inc., San Francisco, 1991.
There are several sources of contaminates in recycled paper. One obvious source of contamination during recycling is the accidental inclusion of foreign material in the paper as it is collected. Another source of contaminants is materials such as binders, adhesives, sealants, glues, etc. These troublesome contaminants can be more generally categorized as "stickies". Their varying sources include polyvinyl acetate and polyvinyl alcohol resins, hot melt adhesives, wet strength residuals, SBR and vinylacrylic rubber lattices, pressure sensitive adhesives, and so forth.
Stickies are so chemically diverse that removal by chemical means alone is very difficult. Stickles interfere with paper production by fouling equipment and reducing the quality of the finished product. Stickies are generally considered to be the product of additions to the paper as contrasted with "pitch" which derives from naturally occurring resinous byproducts in the wood.
Stickies accumulate in white water recycle systems resulting mostly in deposits on paper forming fabrics, on felt and on wet end equipment. Consequently, quality problems such as pinholes, increased down time due to frequent break down and clean ups, additional costs for cleaning and prevention, inherent damages to felt, fabric, and drying equipment are commonly encountered in the production and use Of secondary or recycled fiber furnish. See Moreland, Robert D., "Stickies Control by Detackification." 1986 Pulping Conference, Tappi Press, Atlanta, 1986, p. 193. As will be readily understood, these problems cost mills time and money.
Larger sized (&gt;0.42 mm) stickies generally can be easily removed by mechanical screening and sieving processes and are not a significant problem. Medium sized stickies (0.075-0.42 mm) cannot be effectively removed by mechanical means. They may be pliable making them difficult to remove by screening. Small size stickies are not a problem as long as they remain stable in the furnish. However, once they become unstable, they accumulate, agglomerate, and stick onto surfaces of the papermaking equipment. They eventually grow larger and are subsequently released into the pulp resulting in pinholes, breakages in the sheet, and poor paper quality.
The other class of contaminants in recycled paper addressed by the present invention is ink. Since ink by its nature is colored (usually black), the presence or retention of ink in the formed sheet reduces brightness and can cause dark spots. The increasing use of mixed office waste as a source of recycled fiber for printing and writing grades poses a particular problem in recycling waste paper. Mixed office waste contains a high percentage of nonimpact printed material such as xerographic or laser printed paper that is difficult to deink. The toner particles when removed from the fiber surface have been found to be flat and platelike. Also the density of the separated ink material tends to be about equal to that of the medium which makes removal by conventional mechanical means such as screening, cleaning, flotation and washing difficult. The best solution to ink contamination is to remove the ink prior to paper formation.
Current techniques for stickies control as reported in the general literature can be grossly categorized into mechanical methods and chemical methods. Mechanical methods include combinations of coarse and fine screening, hydrodynamic washing and high intensity dispersion. The high intensity dispersion units readily break the stickies into very small particulates that can be absorbed onto the pulp fiber without adversely impacting the quality of the finished sheet. High consistency pulping combined with agglomeration chemistry also contributes to stickies removal through consolidation of the stickies into large enough mass that enhances easy removal by mechanical means.
Use of chemical additives is a much more common approach to stickies control. Traditional additives include talc, organic solvents, alum, sequestering agents, dispersants (cationic, anionic, and nonionic), zirconium compounds, and organotitanium compounds. See Doshi, Mahendra R., "Properties and Control of Stickies", Recycled Paper Technology, Tappi Press, Atlanta, 1994, p. 73. All these chemical additives have exhibited some, albeit limited, success in the control of stickies. The mechanisms associated with chemical additives involve dispersion, electrostatic attraction, agglomeration, surface tension modification, adsorption, detackification and so forth. Conventional chemical methods for controlling stickies detackify, disperse and retain the stickies in the finished pulp. However, dispersed stickies may accumulate in white water, become unstable, agglomerate and result in poor paper quality and decreased machine performance. Although the objective of preventing the stickies from adhering to equipment is attained, the stickies end up being incorporated into the finished sheet. Also, retained stickies may contribute to increased dirt and lower the brightness of the pulp.
U.S. Pat. No. 4,964,955 to Lamar and other literature teach pitch control processes using cationic polymeric agents and/or chemically modified bentonites and clays in papermaking processes. See also Hassler, Thord. "Pitch Deposition in Papermaking and the Function of Pitch-Control Agents", Tappi Journal, Vol. 71 (6) 1988, p. 195. Lamar, in particular, discloses reducing pitch through the addition of a cationic particulate such as kaolin to the furnish. However, using the Lamar method, the,pitch is not removed but ends up being dispersed and retained in the paper.
U.S. Pat. No. 5,151,155 to Cody teaches a deinking process using a cationic smectite clay. Cody teaches removing the ink waste by either flotation or washing. Cody proposes that it would be highly desirable if the process also removed tacky contaminants. As noted in Moore, D. M. and Robert C. Reynolds, Jr., X-Ray Diffraction and the Identification and Analysis of Clay Minerals, Oxford University Press, Oxford, N.Y., 1989, pp. 119-121, smectite is a group of clay minerals with a layer charge between 0.2 and 0.6 charge per formula unit which swell in the presence of water. Kaolin, in contrast, has approximately a zero layer charge. There are critical nontrivial differences between the process as taught by Cody and the use of cationic kaolins in accordance with the present invention. In particular, suspensions of modified smectite clays cannot be obtained in concentrations substantially greater than about 7.2% solids which makes them expensive to ship, whereas suspensions of cationic kaolins are generally obtained in concentrations greater than about 40%. In addition, the Cody process requires about 26% or about 535 pounds of smectite clay on a dry basis per ton dry pulp. By contrast, the process of the present invention can require as little as 20 pounds of cationic kaolin per ton of pulp on a dry basis.
U.S. Pat. No. 5,131,982 to St. John teaches the use of cationic polymers in effect as a retention aid in the treatment of coated broke. Undesirable components are caused to attach to the fibers of the coated broke after they have been repulped and are retained in the pulp.
U.S. Pat. No. 4,190,491 further teaches the use of cationic polymer as a pitch retention aid. The pitch control additive functions to disperse the pitch and deposit or retain the dispersed pitch into the pulp in finely divided form.
U.S. Pat. No. 5,221,436 discloses use of cationic polyelectrolytes and a clay mineral for pitch control by incorporation. Oleic acid was used to simulate pitch and the effectiveness in a real papermaking system is questionable. This patent does not mention stickies and is of no relevance to the product of our invention.